The U.S. Pat. No. 3,619,549 to Hogan et al No. 3,619,549 discloses a water constricted plasma arc torch for metal cutting, and wherein a swirling flow of water is formed which impinges upon the plasma arc and which serves to constrict the arc and thus improve cutting quality and speed. The U.S. Pat. No. 3,641,308 to Couch et al discloses a similar torch and wherein the water is formed about the arc by an annular nozzle assembly. The nozzle assembly comprises a pair of upper and lower disc like nozzle members which are positioned below the electrode. The plasma gas and arc pass thorough aligned bores in the two nozzle members, and the water is introduced radially toward the plasma gas and arc through a gap which is formed between the two nozzle members.
The U.S. Pat. No. 4,311,897 to Yerushalmy, No. 4,311,897 represents a further refinement of the above basic torch design. More particularly, Yerushalmy teaches that a predetermined dimensional relationship must be maintained between the length of the bores and the water gap defined between the two nozzle members, in order to minimize the sensitivity of the torch to variations in torch "standoff" from the workpiece. The term "standoff" as used herein refers to the particular distance between the end of the bore in the lower nozzle member and the workpiece, and which is determined empirically to maximize the cut quality and speed. As a specific example, the predetermined standoff for cutting at 400 amperes with a model PT-15 torch as presently sold by L-Tec Welding and Cutting 0 Systems of Florence, S.C, is usually 0.375 inches plus or minus 0.06 inch. The Yerushalmy patent teaches that the length of the bore in the lower nozzle member is the most significant factor in controlling standoff sensitivity, and that the optimum range for the length of the lower bore is a fraction of the length of the bore in the upper nozzle member and should lie between 0.07 and 0.16 inches for a torch of the described type. This teaching has been followed by those skilled in the art, and the bores of the upper and lower nozzle members in commercial torches have been designed within the indicated parameters. Also, it has been desirable and commercial practice to fabricate the lower nozzle member from a ceramic insulator material, and it was believed that arc starting would be difficult if not impossible if the length of the lower bore were longer than taught by the Yerushalmy patent.
When making beveled cuts at for example 45.degree., the torch must be inclined with respect to the workpiece and it is impossible to maintain a standoff of less than about 0.62 inches utilizing the above described PT-15 torch, since the body of the torch will contact the workpiece. This very long standoff is significantly beyond the empirically determined most satisfactory distance, and it results in wider kerf widths, rounded cut surfaces, and burn back on the top edge of the workpiece.
It is accordingly an object of the present invention to provide a plasma arc cutting torch of the described type wherein the most effective standoff can be maintained even when the torch is inclined with respect to the workpiece so as to produce a beveled cut.
It is also an object of the present invention to provide a plasma arc torch which is capable of producing beveled cuts of improved quality and speed, and which has acceptable starting characteristics.